The present invention relates to an electrical discharge machining system for machining a workpiece by means of an electrical discharge energy between an electrically conductive workpiece (anode) and a tool, i.e., an electrode (cathode), while the workpiece and the electrode are fed relative to each other.
In such an electrical discharge machining system, the period of each electrical discharge cycle consists of a pre-discharge time, an electrical-discharge time following the pre-discharge time, and a rest time following the electrical-discharge time. In the pre-discharge time, a voltage is applied between the workpiece and the electrode so that an electrical discharge may take place between the workpiece and the electrode when the machining gap therebetween becomes narrower than a critical value. The electrical discharge exists during the electrical-discharge time. During the rest time, no voltage is applied between the workpiece and the electrode.
In an electrical machining system of wire cutting type, for example, optimum values of the electrical-discharge time and the rest time are predetermined depending upon specific machining conditions, such as material of the workpiece, machining depth (height of the workpiece), rate of lengthwise feed of the wire electrode, and desired surface finish of the workpiece. These electrical-discharge time and rest time are indispensable to an electrical discharge machining process. However, the pre-discharge time is not indispensable or essential to the machining process itself. This pre-discharge time is determined in connection with a rate at which the workpiece and the wire electrode are fed relative to each other. Therefore the length of the pre-discharge time, which is a time interval between the start of application of a gap voltage and the start of an electrical discharge, is important for improving the efficiency of machining. For efficient machining, this pre-discharge time which is not fixed, must be kept to a minimum to the extent that the workpiece and the electrode will not contact each other, to prevent electrical short-circuiting of the power supply circuit.
In light of the above, it has been known to detect a voltage at a machining gap between the workpiece and the electrode, e.g., wire electrode, for the purposoe of controlling feeding movements of the workpiece relative to the electrode. The detected gap voltage (voltage data) is applied to a control device such as a central processing unit (CPU), via a filter, an isolator and an A/D converter. The control device compares the detected gap voltage to a predetermined reference voltage, and the difference between the two voltages is used as the input signal to a controller of feeding units to feed the workpiece relative to the electrode for efficient machining operation. Although this feed control method is highly reliable, it needs the use of electrical components or circuits such as the aforementioned filter, isolator and A/D converter, and thereby complicating the electric control device of the machining system. Further, the above method suffers a problem that a contact of the workpiece with the wire electrode makes it impossible to detect the gap voltage and therefore makes it difficult to achieve an accurate control of the feed rate. In the case where the machining operation requires a relatively long rest time after each electrical discharge, using a relatively high feed rate may cause a contact between the workpiece and the electrode. If this happens, the gap voltage falls to substantially zero level, whereby the measurement of the gap voltage for controlling the feed rate is disturbed by noises and other factors resulting from the electrical contact between the workpiece and the electrode. Thus, the prior art method is not capable of controlling the relative feeding movements between the workpiece and the electrode so as to attain maximum machining efficiency of the machining system.